GEOLOGY OF THE SCHROON LAKE QUADRANGLE 77 



Faults and JZones of Excessive Jointing 



General features. The Schroon Lake quadrangle lies in the 

 midst of the faulted eastern Adirondack region. Fifteen earth 

 fractures are represented on the accompanying geologic map. 

 More than likely there are others, but only those which show at 

 least fairly satisfactory evidence for their existence are mapped. 

 In most cases these earth fractures are rather well-defined faults, 

 while in others they appear to be zones or belts of excessive joint- 

 ing in which more or less crushing and minor faulting have taken 

 place. These faults or broken-rock zones are relatively straight 

 for considerable distances, ranging from a mile or two to 10 or 12 

 miles within the quadrangle. Where observations were made, the 

 fault crush-zones are commonly from 25 to 100 feet wide. In 

 accordance with most of the more conspicuous faults of the eastern 

 Adirondacks, those of the Schroon Lake quadrangle mostly strike 

 north-northeast. The topographic influence of the fault zones is 

 usually very striking as a glance at the geologic map will reveal. 

 It is very important to note that the fault zones of weakness, 

 mostly clearly marked by long, narrow valleys, nearly all trend 

 almost or quite at right angles to the strike of the foliation of all 

 the rocks and to the general trend of the belts of relatively weak 

 Grenville which are essentially parallel to the foliation (see map). 



The faults are all of the normal type with fault surfaces vertical 

 or very steep. Within the area of Precambrian rocks of the eastern 

 Adirondacks it is often difficult to demonstrate the existence of 

 faults and, when a given fault has been proved to exist, it is usually 

 difficult or impossible to trace it across country with any great 

 degree of accuracy because of scarcity of exposures due to accu- 

 mulation of glacial and postglacial deposits in the fault valleys. 

 Because of the character and structure of the rock masses (mostly 

 igneous) and the lack of any very clearly defined stratigraphic 

 relations, it is practically impossible to determine the actual amounts 

 of displacements, though in some instances minimum figures can 

 be given. Within the quadrangle such minimum figures are not 

 definitely known to be more than some hundreds of feet, but actual 

 displacements' may have been many times as great. 



Among the more positive criteria for the recognition of the faults 

 and zones of excessive jointing in the quadrangle are the following: 

 (i) long, narrow, almost straight valleys which trend at high 

 angles across the strike of the older rock structures such as the 

 foliation and the belts of Grenville strata; (2) steep to vertical 



